Combine BBQ seasoning and brown sugar in a small bowl. Rub mixture over bacon slices. Arrange bacon on a baking sheet or a broiler pan covered in foil. Bake bacon for 18-20 minutes or until cooked through. Remove and let cool.
Arrange 2 slices of bacon on 4 slices of bread. Top bacon with tomato slices (about 2 slices), followed by crumbled blue cheese and spinach. Top with remaining bread slice and serve.